1. Field of the Invention
This invention relates generally to a method for fabricating semiconductor structures which contain at least a doped region in a compound semiconductor layer of III-V or II-VI elements with Group V or Group VI elements introduced therein, in excess amount, to form either microcrystals or point defects in the doped region and, more particularly, it relates to the fabrication of quantum structures in III-V compound layer such as GaAs. This invention is also directed to the fabrication of an improved FET structure which has a gate length less than 0.1 micron.
2. Description of the Prior Art
Electrons confined in a potential well whose size is comparable to that of the de Broglic wavelength of an electron behave quite differently from those in free space. This special behavior is called the quantum confined effect. One-dimensional plane. Two-dimensional confinement allows only one-dimensional motion of carriers: a structure with this feature is called a quantum wire or quantum well wire. Three-dimensional confinement does not allow kinetic motion of electrons, for example, in any direction. This electronic state is similar to those in atoms. An electron in this state is called a zero-dimensional electron and a structure that gives rise to this state is called a quantum dot or quantum box.
The difference in the dimensionality of these electrons causes quite a big difference in their density of states and electronic energy levels. Moreover, a remarkable improvement can be expected in the performance of devices that use the feature of electron kinematics in quantum structures of less than three dimensions. For example, an electron's mobility is related to the speed of an electronic feature of electron kinematics in quantum structures of less than three-dimensions. For example, an electron's mobility is related to the speed of an electronic device and its power consumption. This mobility is subject to electron scattering processes in semiconductors.
A quantum wire in which an electron has only one-dimensional freedom of kinetic motion allows only forward and backward scattering. Only backscattering reduces the mobility, but the probability of such scattering can be expected to be very low. Extraordinarily high mobility (10.sup.7 -10.sup.8 sqcm/Vs) is predicted in the Japanese Journal of Applied Physics, Vol.19, No.12, December, 1980, pp.L735-L738 in an article entitled "Scattering Suppression and High-Mobility Effect of Size-Quantized Electrons in Ultrafine Semiconductor Wire Structures" by H. Sakaki. It is easy to understand the improvement, if we compare this value with that of bulk GaAs (10.sup.3 sqcm/Vs).
There have been many attempts to make quantum wires and quantum boxes, in the hope of effecting a remarkable improvement in the device performance of electronic devices. However, these attempts have hitherto resulted only in rather large quantum wires whose quantum size effect is insufficient for practical purpose. These quantum wires also present many problems: lack of uniformity, low packing density and difficulty in obtaining a multi-layered structure and cleanliness in the fabrication process, which is important for subsequent crystal growth. Even if a quantum wire has high electron mobility, the current per quantum wire is so small that a large bundle of quantum wires is required to operate an actual electron device. This means that the size uniformity, high packing density, and multi-layered structure are indispensable for quantum wires.
Prior methods of fabricating quantum wires can be classified into four types, as summarized below. Except for molecular beam epitaxy (MBE), or metal organic chemical vapor deposition (MO-CVD), described at 4 below, grown quantum wells or superlattices are used for one-dimensional quantum confinement. Many different methods are used for additional confinement.